CCD imagers of the field transfer type comprise an image (A) register, a storage (B) register and a read-out (C) register. The A-register includes a matrix of photosensitive elements that receive radiant photon energy, i.e., light, from a scene being viewed. The charge proportional to the intensity of the radiant energy is generated, and the accumulated charge is stored for some period of time, less than one field in the case of a television signal. During the vertical blanking interval the accumulated charge is transferred in columns from the A-register to the B-register under the control of clocking signals. During the horizontal blanking interval, one horizontal line of charges is shifted in parallel to the C-register under the control of a clock signal. During the active portion of a horizontal television line the charge packets are serially read out of the C-register under the control of a clocking signal. During the next horizontal blanking interval the next horizontal line is parallel shifted into the C-register from the B-register. Eventually all of the lines stored in the B-register are shifted into and then read out of the C-register. After one field has been read out of the B-register it is ready to receive another field of accumulated charge from the A-register. The invention described herein is not limited to field transfer type of imagers. It would apply to other devices as well, for example, an interline transfer type of imagers.
The output signal from the C-register can be sensed by a floating diffusion of an on-chip charge-to-voltage converter. The output signal which is derived from the floating diffusion is buffered by a MOSFET amplifier which is integrated onto the same chip as that in which the imager is formed to achieve low input capacitance. Such transistors compatible with the CCD fabrication processes, however, have a large 1/f noise, i.e., the noise energy per unit of bandwidth increases as frequency decreases. In particular, this noise is especially troublesome from DC to about 100 KHz and causes random horizontal streaks and flicker in the displayed image. These streaks and flicker are visible under low illumination conditions.
Further, CCD imagers having floating diffusion output stages have an associated reset noise. After a packet of accumulated charge has been sensed, the floating diffusion is reset in preparation to receive the charge during the next clock pulse. The reset noise arises from the reset of the floating diffusion to the reset drain through a conductive channel modulated by thermal noise.
The use of prior art correlated double sampling (CDS) in conjunction with CCD imagers having floating diffusion output stages greatly reduces floating diffusion reset noise and removes low frequency amplifier noise. CDS circuits for use with visible CCD imagers have been built and produce a significant improvement in low light level performance. The resulting high frequency noise, however, which modulates the black reference can be converted to low frequency noise in some systems. In accordance with the present invention a further reduction in the low frequency noise of a CCD imager system is realized.